The Biology of . . . Cheese

Safety vs. flavor in the land of Pasteur

Drive southeast from Dijon in France, across the broad plain of the Saône River, until the land starts to roll and the Jura Massif looms gray on the horizon, and you're in the Franche-Comté--Pasteur country. The great Louis was raised in Arbois, a small town nestled at the base of the Jura cliffs. You can still visit his ivy-covered childhood home, where his father tanned hides and where in later life Louis installed a bathtub (one of the first in town) as well as a laboratory in which to pass his summer holidays. On shelves in the lab stand the flasks of chicken broth--made in 1883 and untouched since--with which Pasteur disproved the theory of spontaneous generation. In the opposite corner is the saunalike room where he tested the effects of heat on microbes. It was Pasteur, of course, who discovered that you could kill microbes by heating them--or "pasteurizing" them, as people would later call it.

To make a Swiss cheese, an American cheese maker begins with raw milk and adds rennet.

(A) Once curds form, they're cut apart and removed by hand

(B) Then the whey is strained off

(C) The curds are molded

(D) The cheese is aged 3 to 12 months to develop its flavor.Photographs by Arthur Meehan

A few hundred yards from that shrine, in the center of Arbois, you can also watch cheese maker Thierry Bobillier make Comté, the Swiss-style cheese that is the pride of the region. Clad in white from cap to rubber boots, in a workshop that is all tile and gleaming stainless steel, Bobillier dips his hand into a 600-gallon vat of warm, butter-yellow liquid--the whey--and pulls out a fistful of white curds, like plump grains of rice. He squeezes them into a ball, stretches them like modeling clay, rubs them between his fingers; now they're ready to be pressed into five 90-pound wheels of Comté. Ask Bobillier what he thinks of pasteurization and you get a hearty laugh. "We're not for it," he says. By French law, he can't make Comté from anything but raw, unpasteurized milk.

Cheese is milk that has been curdled and fermented by microbes. Fermenting microbes are naturally present in raw milk, but pasteurization wipes them out along with the pathogens. In industrial countries today, most cheese is made from pasteurized milk, to which the cheese maker has then added back a few selected microbial cultures to do the fermenting. But artisans like Bobillier, of whom there are many in France and a growing number in the United States, continue to make cheese from raw milk, arguing that its rich natural microflora adds flavor to the cheese. In the United States, pasteurization is required for all fresh or soft-ripening cheeses, but it is still legal to use raw milk for hard cheeses such as cheddar that are aged for at least 60 days.

The Food and Drug Administration, however, is thinking of changing that. Worried by experimental evidence that certain strains of pathogenic bacteria--such as the notorious Escherichia coli 0157:H7--may survive even 60 days of aging, it is considering an outright ban on raw-milk cheese. The American government has already pushed for this position in negotiations for international food standards--the so-called Codex Alimentarius--that are supposed to smooth international trade. It has met, naturally, with vigorous resistance from France, land of 400 cheeses, the finest of which are made of raw milk.

When you eat such a cheese, you are eating an evolving ecosystem. There are billions of bugs in every bite. In a Comté, as in most cheeses, the evolution begins with bacteria that convert lactose--milk sugar--into lactic acid. This sours and curdles the milk: The protein suspended in it, called casein, clumps together to form solid curds that float in the liquid whey. In many cheeses the curdling is hastened by adding rennet, an enzyme preparation taken from the fourth stomach of calves (or made synthetically). Calves curdle their mother's milk so that it can pass through their intestines slowly enough for the nutrients in it to be absorbed. Some 5,000 years ago, humans started making cheese for practical reasons too: They found it takes longer to rot than raw milk does. The good microbes help keep the bad ones in check (although before Pasteur no one knew about microbes).

Meanwhile, those lactic-acid generators foster other microbes that thrive in a sour environment and ripen the cheese, giving it taste and aroma. "You've got bacteria, yeasts, and surface-growing molds," says biochemist Pascal Molimard of Degussa BioActives, which sells cultured microbes to cheese makers. "Those microorganisms break down the proteins and lipids in the curd and use them as nutrients. And so the result is a large number of very small molecules which, being small, are volatile and pass into the atmosphere. It's the mixture of all those molecules that make up the aroma of the cheese."

The mold Penicillium camemberti, for instance, is what gives a Camembert its mottled white rind; it sends enzymes into the interior of the cheese that break down the caseins into smaller molecules, which in turn propagate back to the surface and nourish the mold. In the process the interior of the cheese gradually changes from chalky paste to creeping ooze, and the Camembert, especially a raw-milk one, acquires a fungal aroma reminiscent of old tennis shoes. A closely related mold called Penicillium roqueforti, using similar enzymes, achieves a completely different result: the blue color and tangy "blue" aroma of Roquefort.

Companies like Degussa have cultured a few dozen of these microbes; an ambitious maker of pasteurized cheese might add five or six to the blank slate that is pasteurized milk. "But in a raw-milk cheese, it's not five or six strains you're adding, but a hundred," says microbiologist Eric Beuvier of the National Institute for Agronomic Research lab in Poligny, just down the road from Arbois. "Which makes for a great diversity of flavors." Different microbe strains may break down milk proteins in subtly different ways, producing different precursors of aroma molecules; more strains make for a more complex and distinctive aroma and flavor. Beuvier's team has shown that raw-milk cheeses contain larger and more diverse populations of bacteria than do pasteurized cheeses; tasting panels consistently distinguish raw-milk Comté because of its more powerful and pungent aroma--an aroma that so far only nature can provide.

Of course, E. coli O157:H7 and Listeria monocytogenes are part of nature too. Yet there is next to no evidence that these bugs have caused illness through aged raw-milk cheeses, as they do through contaminated meat. "If you look at the epidemiologic data, there are very few outbreaks that involve cheese at all," says food microbiologist Catherine Donnelly of the University of Vermont. "Particularly the aged hard cheeses appear to be microbiologically safe. Even though cheese consumption is going up, [outbreaks from those cheeses] are just not showing up in the epidemiologic literature."

Ironically, the cheeses that have caused illnesses have often been made from pasteurized milk and then contaminated during processing. "Pasteurization may actually create a more dangerous situation, in that you knock out the competitive flora," Donnelly says. The good bugs that help keep the bad bugs in check in a raw-milk cheese are destroyed by pasteurization.

Donnelly did her study for the Cheese of Choice Coalition, an organization of "farmstead" cheese makers and cheese importers who have joined forces to resist mandatory pasteurization. France, they point out, has managed to avoid mass death-by-cheese by implementing that other great idea of Pasteur's: strict hygiene, both on the farm and at the cheese maker's. "There is a lot of rigor to the work that goes into this product," cheese maker Bobillier told this reporter, who at the time was wearing plastic booties, a gown, and a hairnet. Indeed, French raw milk has gotten so microbe-free of late that government scientists like Beuvier worry that it may be compromising the flavor of cheese. That concern may seem quaint to American authorities, but after all, cheese is supposed to be more than nonlethal. It's supposed to taste good as well.